The three-term senator's campaign office issued a statement Tuesday afternoon that was confirmed by her campaign manager. The Republican senator said in the statement that she was confident she would've won re-election but said she was frustrated by "an atmosphere of polarization and my-way-or-the-highway ideologies."

Snowe has served 33 years in Congress.

Snowe, who's viewed as a moderate, said she sees a "vital need for the political center in order for our democracy to flourish and to find solutions that unite rather than divide us." She said she sees unique opportunities to build support for such change from outside the Senate.

U.S. Sen. Scott Brown, R-Mass., said he will miss Snowe in the Senate.

"I have a great deal of respect for Senator Snowe. She has been a voice of reason in the Senate representing Maine well for many years," Brown said in a statement. "We all are going to miss her independence and her ability to build bridges on some of the toughest issues. Her legacy of service to America in the United States Senate should serve as an inspiration to all of us."